Sometimes you see some of the most obviously painful questions in the forums and you scratch your head, did that person just ask that publicly.
A Google Webmaster Help thread has one person asking if "backlinks from forum profile" can hurt him.
John Mueller from Google responded directly and clearly that it can hurt you. He wrote:
Just to be absolutely clear, if you are dropping links to your site in other people's forums in the hope of gaming search engines, then that's considered web-spam and can be taking into account by both our algorithms and our manual web-spam teams. It doesn't matter how much "PR" the other site has, it doesn't matter if it's a .gov forum -- what you're doing would be considered web-spam by us.
If you care about how search engines like Google view your site, I'd recommend cleaning those link-drops up, removing the link to your site, and not doing that in the future.
One of the more common manual actions I see are those with forum links, to be honest. Just stay away from it.
Forum discussion at Google Webmaster Help.